EVALUATION OF ALTERNATIVE
ASSESSMENT OF RESISTANCE IN TESTS OF PEAS WITH FUSARIUM OXYSPORUM f.
sp. PISI

Stuhler, Ingrid 'Institute of Applied Genetics

and Heidi JaiserFree University Berlin

1000 Berlin 33, West
Germany

For resistance tests of peas with
Fusarium oxysporum f. sp. pisi (Lindf.) Snyder & Hansen
race 1 and 2 normally the degree of wilting is recorded. The wilt index
proposed by Ebbels (2) is widely used. Trying to establish this test we
had considerable difficulty in distinguishing between resistant and
susceptible reactions. Physiological wilting occurred due to the high
temperature necessary for infection, and sometimes the susceptible
cultivars did not develop typical symptoms like greyish green foliage and
downward curling of leaves. Furthermore, the resistant cultivars also had
significantly higher wilt indices than the non-infected controls.
Therefore we evaluated additional or alternative characteristics to
determine resistance or susceptibility.

Five resistant and five susceptible
lines were used: the six host differentials New Season, New Era,
Dark Skin Perfection, WSU 28, WSU 23 and Little Marvel, and the cultivars Rondo, Zem,
Birte and Dik Trom.

Isolates were maintained in the
refrigerator in dry soil. In order to prevent changes of the fungus in
culture a pea extract (150 g peas/l) with a low sugar content (5 g
glucose/l) was used as solid and liquid medium to prepare inoculum. The
liquid culture was filtered after five days of shaking at 100-120 rpm and
adjusted to an inoculum concentration of 106 propagules/ml,
mainly microconidia. Peas were grown in vermiculite for 10 days at a room
temperature of 22°-24°C and a substrate temperature of 24°-26°C and 16 h
light. 10-15 ml of inoculum were added to each plant after the roots had been stabbed by
inserting a scalpel into the substrate around each plant (1).

Symptom development was assessed 18
days after inoculation. Per experiment 24 plants in two replicates were
scored. Visible wilt symptoms were recorded on the key proposed by Ebbels
(2) and an average wilt index was calculated. The vascular discoloration
was observed. Stems of 8 plants were surface sterilized and
cuttings of nodes 1, 4, 5 and 6 (including scale nodes) were
incubated on agar plates for three days at 25°C and the number of colonies
formed was determined. Plant length and number of nodes of all plants were
measured and the mean values calculated.

The results of these tests are
presented in Tables 1 and 2. Although the correlation coefficient was very high
for the wilt indices determined in the two replicates (r = 0.94), some
lines showed different reactions. The relatively high amount of wilting
after infection in the resistant lines cannot be explained. Vascular
discoloration was less influenced by environmental conditions and it was
only observed in the susceptible cultivars. Growth of colonies in stem
sections of the upper nodes (4-6) incubated on agar plates was only
observed in the susceptible lines, whereas in cuttings of node 1 colonies
were also formed in the resistant lines to a certain extent. Stunting of
the infected plants lead to the formation of at least two nodes less and a
reduction of about 20% in plant length. The number of nodes showed less
variation within and between the

replicates.

Of the characters tested the absence or presence of
Fusarium oxysporum

70PNL Volume 21 1989
RESEARCH REPORTS

in the upper stem regions seems
to give the most clear-cut decision on
resistance/susceptibility.